kernel: add request_frames() frame callback + UiSurface::transition_timing()

Two additive primitives for C++-driven, RCSS-tunable animation:
- Host::request_frames(cb) -> FrameRequest: a per-frame callback (RAII handle)
  run before tick_all each frame; the kernel schedules frames continuously while
  >=1 request is alive and stops at rest. Fills the missing animation timer.
- UiSurface::transition_timing(element_id, property): reads the RCSS-authored
  transition duration + easing, returning RmlUi's tween wrapped as a pure
  std::function (no RmlUi types cross the contract) so an extension can drive its
  own animation with hot-reloadable, designer-tunable timing/easing.

0 files changed, 0 insertions, 0 deletions